1. Field of the Invention
The present invention relates to a signal processing circuit of a rotation detector. The present invention also relates to a rotation angle detector.
2. Description of the Related Art
A conventional rotation detector is disclosed, for example, in US 2007/0139036 A (corresponding to JP-A-2007-170992). The rotation detector includes two magnetic sensors and a signal processing circuit. When a rotor having gear teeth rotates, the two magnetic sensors respectively output rotation detecting signals Sa and Sb having different phases. As shown in FIG. 12, the signal processing circuit determines that a rotational direction of the rotor changes based on a change in a phase relationship between the rotation detecting signals Sa and Sb and generates a reverse signal REV. In addition, the signal processing circuit reads all edges of the rotation detecting signal Sa.
The signal processing circuit generates a level-change prohibiting signal for prohibiting changes in a signal level that synchronize with a first rising edge and a first falling edge of the rotation detecting signal Sa after the change of the rotational direction. Based on the level-change prohibiting signal, the signal processing circuit masks the first pulse of the rotation detecting signal Sa after the change of the rotational direction and generates a first output signal OUT1. Based on the first output signal OUT1 and the reverse signal REV, the signal processing circuit generates a second output signal OUT2 that transitions between a high level (H) and a low level (L) while the rotor is rotating in the normal direction and transitions between the high level (H) and a middle level (M) while the rotor is rotating in the reverse direction.
When the first pulse of the rotation detecting signal Sa after each change of the rotational direction is masked, it may be difficult to detect a rotational motion of the rotor with accuracy based on the second output signal OUT2.
For example, in a case where the rotational direction of the rotor changes with a short period, if the first pulse of the rotation detecting signal Sa after each change of the rotational direction is masked, the second output signal OUT2 does not change while the rotational direction of the rotor successively changes. Thus, when the rotational direction of the rotor successively changes, the signal processing circuit is difficult to detect the rotational motion of the rotor with accuracy.
As an example of a rotation angle detector, a crank angle sensor for detecting a crank angle of an engine is disclosed, for example, in JP-A-58-1180908. An example of the conventional crank angle sensor will be described with reference to FIG. 13 to FIG. 15.
A crank angle sensor shown in FIG. 13 includes magnetic sensors 201 and 202, an amplifier 203, a comparator 204, a filter circuit 205, an N type metal-oxide-semiconductor field-effect transistor (MOSFET) 206, and resistors R201 to R204. A rotor 210 is fixed in a crankshaft of an engine. The rotor 210 has top lands (teeth, protruding portions) 211 and bottom lands (recessed portions) 212 alternately provided along an outer periphery. The top lands and the bottom lands are made of magnetic material. The magnetic sensors 201 and 202 may be magnetic resistant elements, for example. The magnetic sensors 201 and 202 are opposite the outer periphery of the rotor 210 so that the magnetic sensors 201 and 202 can detect passage of the top lands 211 and the bottom lands 212.
When the rotor 210 rotates with a rotation of the crankshaft and the top lands 211 and the bottom lands 212 pass in front of the magnetic sensors 201 and 202, magnetic resistances of the magnetic sensors 201 and 202 periodically changes with the passage of the top lands 211 and the bottom lands 212. The magnetic sensors 201 and 202 periodically output analog signals to the amplifier 203 in accordance with the change in the magnetic resistances. The amplifier 203 amplifiers the analog signals output from the magnetic sensors 201 and 202. The comparator 204 compares a voltage Vx of an amplified signal output from the amplifier 203 and a threshold voltage Vy that generates at a middle point between a split resistor R201 and a split resistor R202, and outputs a pulse signal (binarized signal) in accordance with a comparison result.
For example, the pulse signal output from the comparator 204 transitions to a high level when the voltage Vx of the amplified signal is greater than or equal to a threshold voltage Vy and transitions to a low level when the voltage Vx of the amplified signal is less than the threshold voltage Vy, as shown in FIG. 14. The pulse signal from the comparator 204 is input to the filter circuit 205. The filter circuit 205 removes noise component in the pulse signal to a certain degree. The filter circuit 205 may be, for example, a low pass filter such as a CR filter circuit or a high pass filter. The pulse signal from the filter circuit 205 is applied to a gate of the N type MOSFET 206. When the pulse signal is at the high level, the N type MOSFET 206 is activated and electric current flows between a drain and a source through a current limiting resistor R203.
A change in the electric current is detected as a change in a voltage in an electronic control unit (ECU) through a pull-up resistor R204. The ECU calculates time intervals of the changes in the detected voltage based on a clock signal, calculates a rotation number of the rotor 210 based on the time intervals, and calculates a crank angle based on the rotation number. Then, the ECU controls an ignition time of the engine and an injection time of fuel based on the calculated crank angle.
The pulse signal output from the comparator 204 may be affected by a noise signal that enters the crank angle sensor, as shown in FIG. 15. For example, a noise signal may enter the crank angle sensor at a time when a portion of the rotor 210 in front of the magnetic sensors 201 and 202 changes from the top land 211 to the bottom land 212 or from the bottom land 212 to the top land 211, and thereby threshold voltage Vy or the voltage Vx of the amplified signal from the amplifier 203 may fluctuate. In such a case, the comparator 204 may output a pulse signal that successively transitions between the high level and the low level although the comparator 204 should output a pulse signal that keeps the high level. The ECU may include an edge level, that is, a voltage of an edge of the pulse signal, caused by the noise signal in the calculation of the crank angle. Thus, the calculated crank angle may be not accurate.
The noise signal can be removed by increasing a time constant (for example, a CR time constant) of the filter circuit 205. However, if the time constant is increased, a time when the filter circuit 205 outputs the pulse signal to the ECU may be delayed. In a case where a crank angle sensor is used for a high-developed engine control, the crank angle sensor is required to detect a crank angle at high speed. Therefore, it is difficult to increase a time constant of a filter circuit.